1. Field of the Invention
The present invention relates to the preparation of open pore, nanoporous ceramic materials having a high volume of microporous structure.
2. Description of Related Art
Porous materials play a particularly important role in a number of chemical processing industries and applications. Separations based on membranes are critical in such fields as chemical recovery, purification and dehumidification. Porous oxides (e.g. clays, silica, alumina and zeolites) are the materials of choice as catalysts or catalyst supports in chemical and petroleum processing reactions such as hydrocracking, catalytic cracking, hydrodesulfurization, reforming, and polymerization.
With respect to membrane technology, inorganic membranes offer a number of advantages over polymeric membranes which are typically limited to uses at temperatures below about 250.degree. C. These include: i) higher operating temperatures, ii) greater structural integrity and hence the ability to withstand higher pressure differentials and backflushing and iii) improved resistance to corrosion. Porous oxide, (e.g. aluminum oxide) and carbon membranes offer some of these characteristics, but advanced materials are still required for improved strength, toughness, structural integrity, temperature stability, water and oxygen resistance, thermal shock resistance, molecular selectivity to small molecules and gases, and high flux.
Similar considerations apply to clay and metal oxide type catalysts or catalyst supports, particularly as relates to stability and thermal shock resistance at temperatures above about 500.degree. C.
Ceramic materials of the Si--C, Si--N, Si--C--N, Si--B--C, Si--B--N, Al--N, Si--Al--N, B--Al--N and related types appear to offer many of the properties set forth above. However, the sol-gel synthesis methods typically used to prepare porous oxide membranes or catalyst supports are incompatible with the preparation of ceramics of the type described above because of the need to use water in their preparation. Sintering or reactive sintering of these ceramics likewise produces materials with pore sizes of from about 0.1 to about 1000 microns which are non-uniform and generally too large for effective molecular separation and other uses described above. Chemical vapor deposition can produce microporous ceramic layers, but this tends to be an expensive, high temperature process with limited ability to tailor complex ceramic compositions.
Recently, researchers have discovered improved methods for preparing ceramics using ceramic precursors as starting materials. A ceramic precursor is a material, either a chemical compound, oligomer or polymer, which upon pyrolysis in an inert atmosphere and at high temperatures, e.g. above about 700.degree. C., will undergo cleavage of chemical bonds liberating such species as hydrogen, organic compounds, and the like, depending upon the maximum pyrolysis temperature. The resulting decomposition product is typically an amorphous ceramic containing Si--C bonds (silicon carbide), Si--N bonds (silicon nitride) or other bond structures which will vary as a function of the identity of the ceramic precursor, e.g. Si--C--N, Si--N--B, B--N, Al--N and other bond structures, as well as combinations of these structures. Crystallization of these amorphous ceramic products usually requires even higher temperatures in the range of 1200.degree.-1600.degree. C.
The pyrolysis of various ceramic precursors e.g. polycarbosilanes, polysilanes, polycarbosiloxanes, polysilazanes, and like materials at temperatures of 1300.degree. C. and higher to produce ceramic products, e.g. silicon carbide and/or silicon nitride, is disclosed, for example, in M. Peuckert et al., "Ceramics from Organometallic Polymers", Adv. Mater.2, 398-404 (1990). The pyrolysis of polyorganosilazanes under ammonia atmosphere at pyrolysis temperatures up to 1400.degree. C. is also disclosed in Han et al., "Pyrolysis Chemistry of Poly(organosilazanes) to Silicon Ceramics", Chem. Mater., Vol. 4, No. 3, pp. 705-711 (1992).
During pyrolysis, preceramic precursors such as described above liberate various gaseous decomposition species such as hydrogen and organic compounds, including methane, higher molecular weight hydrocarbon molecules, lower molecular weight precursor fragments and H--C--N species. These gases tend to coalesce within the preceramic matrix as they form, resulting in a bulking or swelling of the mass. These entrained gases can lead to the formation of gas bubbles within the developing ceramic mass as the preceramic precursor crosslinks and hardens, resulting in a lower density ceramic having a voluminous, macroporous or mesoporous closed-cell structure, without development of a significant amount of open celled micropores.
In copending applications Ser. No. 08/248,290, now U.S. Pat. No. 5,643,987, and Ser. No. 08/248,291, now U.S. Pat. No. 5,563,212, each filed in the United States on May 24, 1994, it is disclosed that microporous ceramics can be achieved by the pyrolysis of a preceramic intermediate composition based on an intimate mixture of from about 30 to 99 parts by weight of a preceramic precursor polymer or oligomer and correspondingly from about 1 to 70 parts by weight of a particulate material having a particle size of less than 10 microns. In this process, pyrolysis is conducted at temperatures of up to less than about 1100.degree. C. under flowing inert gas such as helium, argon or nitrogen, or under ammonia gas. Those inventions were based on the theory that the presence of a particulate filler in the preceramic matrix served to prevent the formation of large bubbles of decomposition gases as they were generated during decomposition under inert or ammonia gas, thereby yielding a microporous structure in the pyrolyzed product rather than a voluminous, macroporous mass of low bulk density which was achieved where pyrolysis was conducted under inert gas and the particulate material was not present in the precursor.
Also, copending U.S. application Ser. No. 08/385,299, now U.S. Pat. No. 5,696,217, filed Feb. 10, 1995 as a continuation-in-part of application Ser. No. 08/248,289, filed May 24, 1994, discloses that microporous ceramics can be achieved without the need to include particulate material in the pre-ceramic composition by conducting the pyrolysis at a controlled rate of heating and under flowing ammonia gas and at maximum heating temperatures of less than about 1100.degree. C., preferably less than 1000.degree. C.
In copending U.S. application Ser. No. 08/579,444, filed on Dec. 27, 1995, microporous ceramic materials are disclosed which are prepared by first forming a composite intermediate comprising a colloidal dispersion of a preceramic precursor polymer mixed with discrete, nanoscale metal particles having a dimension of from about 10 to about 500 Angstroms and gradually heating the mixture in the presence of an inert or reactive gas to a temperature of about 300.degree. C. up to less than 1100.degree. C. to achieve a microporous ceramic having a surface area in excess of 70 m.sup.2 /gm and a volume of open pore micropores of greater than about 0.03 cm.sup.3 /gm. Similar microporous ceramic materials are also disclosed in allowed U.S. copending application Ser. No. 08/578,084, filed on Dec. 27, 1995, which are prepared by first forming a composite intermediate comprising a mixture of a preceramic precursor polymer and from about 0.5 up to about 65 wt % of an organometallic compound containing a metal of Group bI, II, III, IV, V, VIB, VIIA, or VIII of the Periodic Table, including Rare Earth metals, and gradually heating the mixture in the presence of a reactive or inert gas to a maximum temperature in the range of from about 300.degree. C. up to less than 1200.degree. C.
It is to be noted, however, in each of the applications discussed above, heating of the preceramic composition is conducted either in an inert gas or ammonia over the entire range of temperature from room temperature up to the maximum heating temperatures disclosed.
The preparation of porous ceramics particularly useful for membrane gas separation processes is disclosed by K. Kusakabe et al., "Preparation of Supported Composite Membrane By Pyrolysis of Polycarbosilane for Gas Separation at High Temperature", J. Membrane Sci. 103, 175-180 (1995). This reference describes the synthesis of a porous membrane structure by pyrolysis of polycarbosilane film, deposited from xylene solution onto a .gamma.-alumina film on the outer surface of an .alpha.-alumina tube, and bulk porous material made by pyrolysis of polycarbosilane, formed by evaporating a xylene solution. After heating to 200.degree. C. in air and holding at that temperature for one hour, pyrolysis was conducted by heating for two hours in nitrogen or air at a temperature in the range of 350.degree. to 550.degree. C., followed by cooling to room temperature. Data are shown indicating the pore sizes are between 20 .ANG. and 100 .ANG.. The reference does not specifically describe the preparation of porous ceramics having a significant content of micropore volume having pore dimensions of less than 20 .ANG. (2 nanometers).